In the manufacturing process of a secondary battery (for example, a lithium-ion battery), a liquid-injection hole on the top cover of the case needs to be sealing welded by laser after completion of the liquid-injection process, so as to prevent leakage of the electrolyte.
In the related art, as shown in FIGS. 1-2, the specific implementation process includes: firstly plugging the liquid-injection hole 20 with a T-shaped sealing nail so as to prevent the electrolyte from flowing out of the hole and thus contaminating the welding seam, then cleaning the electrolyte remaining on the surface of the welding area, and finally sealing the welding portion 400 between the sealing cover-plate 40 and the liquid-injection hole 20 in a manner of laser full-length welding. A full-length welding seam is formed after completion of the welding.
Stress will be formed in the welding process due to thermal expansion and contraction, and the welding stress will remain in the welding seam if it cannot be released, and the welding seam is a flat annular welding seam, thus the welding stress concentration phenomenon is obvious, when the welding stress reaches a certain extent, the welding seam will crack to form a welding crack. At present, as for the full-length welding, the problem of welding crack caused by the welding stress has not been completely solved till now, in particular, tiny cracks cannot be easily detected, which may easily causes liquid leakage, and thus there is a great potential safety risk.
Principle of formation of the welding crack is: the thermal effect of welding leads to welding internal stress in the welding seam, in the full-length welding process, when the welding seam contains relatively more impurities having low melting points, the impurities tend to accumulate at the center of the welding seam to form a liquid film, and under the action of a relatively greater tensile stress, the welding seam will crack and form welding cracks.